A child that age, does know that a knife, is dangerous.
They also know, that these things are not allowed in school.
There are clearly defined, laws/rules about students not being allowed at all, to bring things like this to school.
Every year, per my kids' school, the parents are sent home a several page flier... about "weapons" in schools. Even pretend, weapons.
Your child is 9, and KNOWS this.
Kids are talked to about this, in school.
My daughter is 9. She is in 4th grade.
She clearly... KNOWS the dangers of these things.
Your son should too.
The fact, that he is lying and kept this knife secret, shows that he knows this is a "taboo" item and a dangerous item.
You did not say what he is sneaking... to school.
Or what he is lying about.
Or who his friends are.
Or what crowd he hangs out with.
PERHAPS.... maybe your son "sneaks" things, because he knows his Step-Dad is... unapproachable? Or maybe he cannot be open with his Step-Dad? Is this a possibility?
Does your son tell YOU things?
You said "we found a small knife his friend had given him in his room." But YOU did not find it. Your Husband did. You were at work when you found out.
So how did your Husband find it? Did he search your son's room or backpack?
It seems.... that the knife was hidden... and then your Husband found it.
Your son is a Tween.
9-12 years old, is the preteen ages.
Google search this. "Tween boy development."
If your son, cannot "trust" you or his Step-Dad... then, that will automatically, shut-down any possible communication between all of you. And there will not be a symbiotic relationship between all of you.
And your son will lie and/or hide things from all of you.
Or, if he is getting into the wrong bunch of friends and activities, then it is up to you, to STEER him, the right way.
But not only through parental threats.
One way my late Dad, kept tabs on us and "disciplined" us when we were teens is... he KEPT a good relationship with us. AND had our friends over. AND did not keep us at arm's length. He kept us, close to him... as a parent. He was not our buddy... but a strong parent who lead.
AND who, knew us and with whom we could go to for anything... good or bad, and we could tell him ANYTHING. We never had to lie to him.
We trusted him, as kids. And he trusted us.
Not all kids... sneak things and/or lie.
No matter what age.
Giving your son the ice treatment... is really immature.
And telling your son he is not going to forgive him as easily as his mom... is really, immature as well.
That is only, teaching your son, RESENTMENT and distrust. And how to keep rebelling... against his Step-Dad.